cannot unmount external drive to repair

Hello. I'm having a problem unmounting an external hard drive. I have a 2010 iMac with a 1.5 TB Seagate external hard drive I use for Time Machine as well as other backups. I cannot transfer files to/from this hard drive anymore, and Time Machine will no longer work. I can see the drive in Finder, but some times the folders appear to be empty of files, and sometimes I can see the files but cannot access. Disk Utility states the disk needs to be repaired, but when I try to repair the disk, "unable to unmount for repair" is stated. I've tried to unmount through disk utility as well as through Terminal. I've tried to unmount the entire disk as well as the individual partitions. Any suggestions would be great. Thanks.
I tried to see what's running on the external hard drive and this is what I came up with. Not sure how to proceed from here:
Macinto9e3bfb3c:~ howardhonig$ sudo lsof /dev/disk1s3
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
mds 33 root 16r DIR 1,6 850 2 /Volumes/HLH1.5TB
mds 33 root 17r DIR 1,6 850 2 /Volumes/HLH1.5TB
mds 33 root 19r DIR 1,6 850 2 /Volumes/HLH1.5TB
mds 33 root 25r DIR 1,6 850 2 /Volumes/HLH1.5TB
mds 33 root 26r DIR 1,6 850 2 /Volumes/HLH1.5TB
revisiond 68 root 12r DIR 1,6 850 2 /Volumes/HLH1.5TB
revisiond 68 root 13r DIR 1,6 306 7040379 /Volumes/HLH1.5TB/.DocumentRevisions-V100